The present application relates to grounding assemblies for electronic equipment and particularly relates to grounding assemblies for use with outdoor electronic equipment that is subject to being damaged by lightning. An example of such electronic equipment is the electronic controllers used on irrigation systems, such as those used on golf courses and the like. Irrigation systems include numerous sprinkler heads located throughout a property which are turned on and off by a plurality of solenoid valves located at or near the valves. The solenoid valves have control boards which are typically connected to a central control computer and a power source by wires buried under ground. Lightning striking the ground far from a particular control board can induce voltage spikes in the wires leading to the control board that can destroy the board. Lightning arrestors are typically incorporated in such equipment to prevent this but for such arrestors to protect the equipment adequately they must have an effective connection to ground.
It is the responsibility of the installer to connect all electronic irrigation equipment to earth ground in accordance with Article 250 of the National Electrical Code (NEC.) Grounding, bonding, and shielding components will, at a minimum, include the following items. Earth grounding must be done with grounding electrodes that are UL listed or manufactured to meet the minimum requirements of Article 250.52 of the 2008 edition of the NEC. At the very minimum, the grounding circuit will include a copper clad steel ground rod, a solid copper ground plate installed under ground and in contact with a suitable amount of an earth contact material, such as the carbon backfill products sold under the trademarks PowerSet or PowerFill by Loresco International of Hattiesburg, Miss. This is the minimum requirement for supplementary grounding of any electronic equipment.
FIGS. 1 and 2 illustrate the components required for supplementary grounding. The electronic equipment, such as an irrigation controller, is shown at 10. A ground rod 12 has a minimum diameter of ⅝ inches and a minimum length of 10 feet. The ground rod 12 is driven into the ground in a vertical position or an oblique angle not to exceed 45 degrees at a location 10 feet from the electronic equipment 10, the ground plate 14, or the wire 16 connecting the ground plate 14 to the equipment 10, as shown in FIG. 1. A 6 AWG solid bare copper wire 18 (about 12 feet long) is connected at 19 to the ground rod 12 by the installer using an exothermic welded connection, such as that provided by the Cadweld® GR1161G “One-Shot” welding kit available from Erico International Corporation of Solon, Ohio. The wire 18 shall be connected to the electronic equipment's ground lug.
The copper ground plate 14 must meet the minimum requirements of Article 250.52(A)(7) of the 2008 NEC. It is made of a copper alloy intended for grounding applications and has minimum dimensions of 4 in.×96 in.×0.0625 in. A 25-foot continuous length (no splices allowed unless using exothermic welding process) of 6 AWG solid, round, bare copper wire 16 is attached to the plate by the manufacturer using an approved welding process. This wire 16 is also connected to the electronic equipment's ground lug. In the past the round wire 16 has sometimes been replaced with a braided copper strap for connecting the electronic equipment to the ground plate 14. But braided copper straps have complicated geometry that contributes to higher inductance characteristics. The ground plate 14 is to be installed to a minimum depth of 30 inches, or below the frost line if the frost line is lower than 30 inches, at a location 8 feet from the electronic equipment 10 and underground wire 18. A suitable amount of earth contact material 20 must be spread so that it surrounds the copper grounding plate 14 evenly along its length within a 6 inch wide trench. Salts, fertilizers, bentonite clay, cement, coke, carbon, and other chemicals are not to be used to improve soil conductivity because these materials are corrosive and will cause the copper electrodes to erode and become less effective with time.
The grounding circuit components are to be installed in straight lines, to the extent possible, with no sharp turns. To prevent the electrode-discharged energy from re-entering the underground wires, all electrodes are installed away from such wires. The spacing between any two electrodes is as shown in FIGS. 1 and 2, so that they don't compete for the same soil.
The earth-to-ground resistance of this circuit is to be no more than 10 ohms. If the resistance is more than 10 ohms, additional ground plates and earth contact material are to be installed in the direction of an irrigated area at a distance of 10 feet, 12 feet, 14 feet, etc. It is required that the soil surrounding copper electrodes be kept at a minimum moisture level of 15% at all times by dedicating an irrigation station at each controller location. The irrigated area should include a circle with a 10-foot radius around the ground rod 12 and a rectangle measuring 1-foot×24-feet around the plate 14. All underground circuit connections are to be made using an exothermic welding process by utilizing products such as the Cadweld® “One-Shot” kits. Solder cannot be used to make these connections.
The above grounding circuit is referred-to as supplementary/auxiliary grounding in the NEC. For safety reasons the NEC requires that all supplementary grounds be bonded to each other and to the service entrance ground (power source). This is also the recommended practice of IEEE Standard 1100-1999. Note that this is in addition to the equipment ground, which is commonly referred to as “the green wire.” The black (line or hot), white (neutral), and green wires must always be kept together in a trench, conduit, tray or the like. The bonding conductors are to be 6 AWG solid bare copper unless the system power conductors are larger than 1/0 AWG, in which case they are to be 4 AWG solid bare copper. All splices to the bonding conductors shall be made using an exothermic welding process.